1. Field of the Invention
The invention relates generally to radiolocation, and more particularly relates to a short-range radiolocation system suitable for smaller areas, indoor applications, and cost-critical products such as asset and personnel tags for providing high accuracy position data for situations where Global Positioning Satellite (GPS) systems are either ineffective or too expensive.
2. Discussion of the Related Art
The Global Positioning System (GPS) is a satellite-based navigation system known to be of great utility in many wide-area, outdoor scenarios, particularly in military and commercial navigational applications. This system lets a user with a GPS receiver determine his or her location on the earth with a high degree of accuracy, based on signals received from satellites orbiting the earth. Although this system was developed primarily for military use by the United States Department of Defense, civilian uses have exploded in recent years.
In certain navigational and positioning applications, e.g. industrial, military, transportation, and emergency assistance, there is a need for accurately determining the location of personnel, equipment, containers, personnel, and other assets in smaller areas such as plant buildings, warehouses, staging areas, storage facilities, and production line areas. The GPS generally has poor coverage inside buildings, under forest canopies or heavy foliage, or in highly developed urban areas where tall structures dominate. Multi-path effects of the radio signals from the satellites seriously deteriorate GPS accuracy in situations involving tall structures such as skyscrapers. Furthermore, known GPS receiving hardware is currently too large and costly for mass implementation applications, for example, where there is a need for identifying and locating unattended assets such as an item of equipment, a pallet or container of goods, a vehicle, etc.
GPS operates to determine the position of a user with a receiver by receiving signals transmitted by a plurality of GPS satellites orbiting the earth. The user""s position on the surface of the earth is calculated relative to the center of the earth by triangulation based on signals received from multiple (usually 4: or more) GPS satellites. The distance from the user to a satellite is computed by measuring the propagation time required for a direct-sequence spread-spectrum xe2x80x9cranging codexe2x80x9d signal transmitted by a satellite to reach the receiver.
A ranging code is a pseudorandom code sequence that is generated by a polynomial generator according to a known algorithm, each bit of which is called a xe2x80x9cchipxe2x80x9d to distinguish it from the true data bits that might form a message encoded onto the ranging code. A xe2x80x9cchipxe2x80x9d is a single bit in a pseudorandom code sequence used to spread the spectrum of an information signal. The pseudorandom ranging code sequence, when broadcast by radio, has a spectrum that has widely dispersed sidebands relative to the carrier frequency, and thus is referred to as a xe2x80x9cspread-spectrumxe2x80x9d signal. Spread-spectrum signals are known to have desirable characteristics for data security and resistance to radio-frequency (RF) interference.
Within a GPS receiver, an identical ranging code signal is generated and shifted in time (or phase) until it achieves correlation with the specific satellite-generated ranging code being acquired. The magnitude of the time shift of the identical ranging code signal within the receiver relative to the satellite transmitted ranging code provides a time differential that is related to the satellite-to-user range.
To determine user position in three dimensions, range measurements are made to a plurality of satellites, resulting in four simultaneous ranging equations that have four unknowns. These equations can be solved by computer systems to determine the values of x, y, z (the 3-dimensional location of the user""s receiver), and t, which is a clock error. There are several closed-form solutions furnished in the literature for solving the equation to determine the unknown quantities.
The positioning is in general accomplished by determining the time-of-flight of the signals from at least 4 GPS satellites, and by careful processing of the real-time data from the multiple satellite clocks (and other, small corrections) the actual distances are computed; the common solution to the set simultaneous distance equations, coupled to the known satellite locations, provides the GPS receiver""s position. Thus, the geometric range is given by:
r=c(Tuxe2x88x92Ts)=cxcex94t,xe2x80x83xe2x80x83(1)
where
Ts=system time when signal left the satellite;
Tu=system time when signal reached the receiver;
xcex4t=offset of satellite clock from system time;
tu=offset of receiver clock from system time;
Ts+xcex4t=satellite clock reading when signal left satellite;
Tu+tu=receiver clock reading when signal, arrived;
c=speed of light;
(xu, yu, zu)=position of the receiver in 3 dimensions; and
(xj, yj, zj)=3-dimensional position of the jth satellite (j=1 to 4).
In these terms, the pseudorange is given by:
xcfx81=c[(Tu+tu)xe2x88x92(Ts+xcex4t)]=c(Tuxe2x88x92Ts)+c(tuxe2x88x92xcex4t)=r+c(tuxe2x88x92xcex4t)xe2x80x83xe2x80x83(2)
and the 4 pseudoranges are thus:
xcfx811=[(x1xe2x88x92xu)2+(y1xe2x88x92yu)2+(z1xe2x88x92zu)2]xc2xdctuxe2x80x83xe2x80x83(3)
xcfx812=[(x2xe2x88x92xu)2+(y2xe2x88x92yu)2+(z2xe2x88x92zu)2]xc2xdctuxe2x80x83xe2x80x83(4)
xcfx813=[(x3xe2x88x92xu)2+(y3xe2x88x92yu)2+(z3xe2x88x92zu)2]xc2xdctuxe2x80x83xe2x80x83(5)
xcfx812=[(x4xe2x88x92xu)2+(y4xe2x88x92yu)2+(z4xe2x88x92zu)2]xc2xdctuxe2x80x83xe2x80x83(6)
These nonlinear equations may be solved by either closed-form methods, iterative techniques based on linearization, or by Kalman-filtering (estimation) algorithms.
Although GPS radiolocation is proven and works well, it requires multiple readings to obtain positioning accuracy down to the 10-meter range, which leads to greater complexity in the receiver and longer computation time to calculate a navigational xe2x80x9cfixxe2x80x9d. Costly military receivers are not hampered by these limitations, but commercial receivers do not have the special encryption features required for rapid high-accuracy location determination, although recent advances in signal processing have somewhat improved the situation. In addition, GPS provides location information for receivers in the field which are typically attended by personnel. GPS does not readily adapt to situations where a central locating system is required for locating unattended assets or personnel that cannot respond by transmitting the GPS-determined location information to the central locating system via separate communication means. For these reasons, GPS is generally not suitable for radiolocation in a limited space at low cost for unattended assets. Furthermore, due to the extremely low signal strengths of the GPS satellite beacon transmitters at the GPS receiver, GPS signals are virtually always unusable indoors because of the additional attenuation of the overhead satellite signals by building roofs, upper-floors, and other overhead structures, as well as trees and dense foliage in general. In addition, in xe2x80x9curban canyonsxe2x80x9d and very rugged terrain, often there are too few GPS satellites in direct line-of-sight view of the receiver to obtain a sufficiently accurate position fix.
Further details of the GPS are provided in U.S. Pat. No. 4,894,662, to Counselman, xe2x80x9cMethod and System for Determining Position on a Moving Platform, Such as a Ship, Using Signals for GPS Satellites.xe2x80x9d Further details of the GPS are also provided in Kaplan(1).
One approach to a positioning system for radiolocation in a limited space is a system constructed by PinPoint Corporation and described in an article by Werb(2). This article describes a local positioning system that subdivides the interior of a building into cell areas, and receives a 5.8-GHz tag response signal that is utilized to locate a tag attached to an object such as a medical records file. The tags are small and light for the widest applicability, inexpensive and therefore much simpler in design than GPS receivers. The system as a whole is purportedly capable of tracking thousands of tags to an accuracy of about 10 meters. Certain aspects of GPS have been employed in the PinPoint system. The PinPoint tags are designed to transmit a code for simultaneous arrival at three receivers installed in a facility. In this system, the tags do not include sophisticated circuitry and software for decoding a signal received from a satellite. Rather, the tags simply change a received signal from a transmitter located in the facility and transpond or repeat it back to a receiver with tag identification (ID) information phase-modulated onto it. The receiver extracts the tag ID from the return signal and determines the tag""s distance from the antenna by measuring the round trip time of the signal""s flight. Since the reader generates the signal, there is no need to calibrate a clock in the tag. Since the distance of each reader is determined independently, there is no need to synchronize clocks on various readers.
One particular drawback of the PinPoint system is that the tags, being predominantly passive transponders, only emit about one milliwatt of RF power, so that the tag can only be detected reliably up to a range of about 30 meters. This greatly limits the size of the area in which the system is operative, and requires a multiplicity of receivers if greater coverage is desired, thereby adding to the cost and complexity of the system. Furthermore, this system uses two discrete carriers in two different bands, one at xcx9c2.45 GHz and one at xcx9c5.8 GHz. This scheme thus requires two discrete RF systems, one for each band, which increases the likelihood of interference and requires the simultaneous use of (and xe2x80x9cties upxe2x80x9d) two unlicensed radio transmission bands in the local area. Furthermore, the relative complexity of the PinPoint tag is high, due to the requirement for RF hardware in two widely separated bands.
Although the PinPoint system may be suitable for certain limited applications such as asset location within buildings, there is still a need for a system that is operative over a larger area, e.g. industrial settings, forests, warehouses, staging areas, etc., but does not unduly multiply system complexity and expense in scaling up. One approach to extending the ranging capability beyond that of the PinPoint system is the hybrid-ranging system described by Dixon(3). In this technique, a code sequence of a few thousand chips is further encoded with a digital xe2x80x9crange messagexe2x80x9d whose bit rate is a multiple of the repetition rate of the code sequence which is transmitted as the ranging signal.
Stated in other words, the Dixon approach involves use of a phase-shift keying (PSK) modulated short polynomial code sequence and a superimposed frequency-shift keying (FSK) digital range message whose bit rate is the repetition rate of the polynomial sequence. In general, there is an ambiguity in range caused by using a polynomial code that is too short to handle the full desired distance range, but this is resolved in the Dixon approach by using the relatively slow digital pattern (whose length is such that its repetition period is longer than the promulgation of the delay of the longest range to be measured) to count basic-code repetitions during the two-way signal propagation interval. This system thus involves measuring the relative phase of the received range messages, which is augmented by counting the number of polynomial code repetitions (low-frequency bit alternations), which serve as range message markers. Although this technique is suitable for providing larger total range readings than with the polynomial code by itself, Dixon does not even suggest how to provide any finer measurement resolution.
Accordingly, there is still a need for a radiolocation system that is operative in a larger area than that of a passive tag transponder system and utilizes spread-spectrum signals for security and minimal interference, but is still capable of providing sufficiently accurate ranging at low cost. There is also still a need for a more limited-scale radiolocation system for applications where the expense and complexity of GPS cannot be justified, or where or the technical limitations of GPS are a problem, such as in forest areas, cities with tall buildings, industrial settings, underground, and the like. There is also a need for a low-cost radiolocation system that provides sufficient gross ranging, with unambiguous intermediate and fine ranging resolution capability for greater accuracy when required, which will operate in the relatively stringent bandwidth allocations permitted by law. Finally, there is also a need for a radiolocation system where tags for unattended assets can be manufactured with as many components as possible on a single integrated-circuit (IC) chip, for low cost, small size, low power, high reliability, and good repeatability.
It is an object of the invention to provide accurate positioning information for an RF tag or other device in indoor, underground, or other constrained environments. It is another object of the invention to provide high immunity to multipath propagation effects and other forms of RF noise and interference. It is another object of the invention to provide high positioning accuracy utilizing substantially less RF transmission bandwidth than conventional spread-spectrum ranging techniques achieving similar positioning resolution. It is another object of the invention to alternatively provide high positioning accuracy with substantially shorter measuring times than conventional spread-spectrum ranging techniques achieving similar positioning resolution. It is a further object of the invention to permit radiolocation functions to occur concurrently with, and transparently to, the robust transmission of digital device or tag data over the same spread-spectrum RF link.
One embodiment of the invention is based on a method of determining a location of a tag, comprising: developing a coarse ranging of the location of the tag by determining a phase of a spread-spectrum code sequence that is transmitted by the tag to the plurality of receivers by modulating a carrier with a spread-spectrum code; developing an intermediate ranging of the location of the tag by determining a phase of a difference signal that is transmitted by the tag to the plurality of receivers; utilizing the coarse ranging and the intermediate ranging of the location of the tag to determine a set of distances from the tag to each of the plurality of receivers; and utilizing the set of distances to triangulate a position of the tag. Another embodiment of the invention is based on an apparatus, comprising: a tag including a spread spectrum transmitter; and a plurality of receivers including circuitry to develop a coarse-resolution range value of the location of the tag by determining a phase of a spread spectrum code sequence transmitted by the tag to a plurality of receivers; circuitry to develop an intermediate-resolution range value of the location of the tag by determining a phase of a difference signal that is transmitted by the tag to the plurality of receivers; resources utilizing the coarse-resolution and intermediate-resolution range values of the location of the tag to determine a set of distances from the tag to each of the plurality of receivers; and resources utilizing the set of distances to triangulate a position of the tag. Another embodiment of the invention is based on an apparatus for determining a location of a tag, comprising: circuitry to develop a coarse-resolution range value of the location of the tag by determining a phase of a spread spectrum code sequence transmitted by the tag to a plurality of receivers; circuitry to develop an intermediate-resolution range value of the location of the tag by determining a phase of a difference signal that is transmitted by the tag to the plurality of receivers; resources utilizing the coarse-resolution and intermediate-resolution range values of the location of the tag to determine a set of distances from the tag to each of the plurality of receivers; and resources utilizing the set of distances to triangulate a position of the tag.